Acetohydroxyacid synthase (AHAS; EC 4.1.3.18; acetolactate synthase (ALS)), encoded by the Als nucleic acid, is the first enzyme that catalyzes the biochemical synthesis of the branched chain amino acids valine, leucine, and isoleucine (Singh B. K., 1999, Biosynthesis of valine, leucine and isoleucine in: Singh B. K. (Ed) Plant amino acids. Marcel Dekker Inc. New York, N.Y. Pg 227-247). AHAS is the site of action of four structurally diverse herbicide families including the sulfonylureas (LaRossa R A and Falco S C, 1984, Trends Biotechnol. 2:158-161), the imidazolinones (Shaner et al., 1984, Plant Physiol. 76:545-546), the triazolopyrimidines (Subramanian and Gerwick, 1989, Inhibition of acetolactate synthase by triazolopyrimidines in (ed) Whitaker J R, Sonnet P E Biocatalysis in agricultural biotechnology. ACS Symposium Series, American Chemical Society. Washington, D.C. Pg 277-288), and the pyrimidyloxybenzoates (Subramanian et al., 1990, Plant Physiol. 94: 239-244). Imidazolinone and sulfonylurea herbicides are widely used in modern agriculture due to their effectiveness at very low application rates and relative non-toxicity in animals. By inhibiting AHAS activity, these families of herbicides prevent further growth and development of susceptible plants including many weed species. Several examples of commercially available imidazolinone herbicides are PURSUIT® (imazethapyr), SCEPTER® (imazaquin) and ARSENAL® (imazapyr). Examples of sulfonylurea herbicides are chlorsulfuron, metsulfuron methyl, sulfometuron methyl, chlorimuron ethyl, thifensulfuron methyl, tribenuron methyl, bensulfuron methyl, nicosulfuron, ethametsulfuron methyl, rimsulfuron, triflusulfuron methyl, triasulfuron, primisulfuron methyl, cinosulfuron, amidosulfuron, fluzasulfuron, imazosulfuron, pyrazosulfuron ethyl, and halosulfuron.
Due to their high effectiveness and low toxicity, imidazolinone herbicides are favored for application by spraying over the top of a wide area of vegetation. The ability to spray an herbicide over the top of a wide range of vegetation decreases the costs associated with plantation establishment and maintenance, and decreases the need for site preparation prior to use of such chemicals. Spraying over the top of a desired tolerant species also results in the ability to achieve maximum yield potential of the desired species due to the absence of competitive species. However, the ability to use such spray-over techniques is dependent upon the presence of imidazolinone tolerant species of the desired vegetation in the spray over area.
Among the major agricultural crops, some leguminous species such as soybean are naturally tolerant to imidazolinone herbicides due to their ability to rapidly metabolize the herbicide compounds (Shaner and Robson, 1985, Weed Sci. 33:469-471). Other crops such as corn (Newhouse et al., 1992, Plant Physiol. 100:882-886) and rice (Barrett et al., 1989, Crop Safeners for Herbicides, Academic Press New York, pp. 195-220) are susceptible to imidazolinone herbicides. The differential sensitivity to the imidazolinone herbicides is dependent on the chemical nature of the particular herbicide and differential metabolism of the compound from a toxic to a non-toxic form in each plant (Shaner et al., 1984, Plant Physiol. 76:545-546; Brown et al., 1987, Pestic. Biochem. Physiol. 27:24-29). Other plant physiological differences such as absorption and translocation also play an important role in sensitivity (Shaner and Robson, 1985, Weed Sci. 33:469-471).
Crop cultivars tolerant to imidazolinones, sulfonylureas, and triazolopyrimidines have been successfully produced using seed, microspore, pollen, and callus mutagenesis in Zea mays, Brassica napus, Glycine max, and Nicotiana tabacum (Sebastian et al., 1989, Crop Sci. 29:1403-1408; Swanson et al., 1989, Theor. Appl. Genet. 78:525-530; Newhouse et al., 1991, Theor. Appl. Genet. 83:65-70; Sathasivan et al., 1991, Plant Physiol. 97:1044-1050; Mourand et al., 1993, J. Heredity 84:91-96). In all cases, a single, partially dominant nuclear gene conferred tolerance. Four imidazolinone tolerant wheat plants were also previously isolated following seed mutagenesis of Triticum aestivum L. cv Fidel (Newhouse et al., 1992, Plant Physiol. 100:882-886). Inheritance studies confirmed that a single, partially dominant gene conferred tolerance. Based on allelic studies, the authors concluded that the mutations in the four identified lines were located at the same locus. One of the Fidel cultivar tolerance genes was designated FS-4 (Newhouse et al., 1992, Plant Physiol. 100:882-886).
Computer-based modeling of the three dimensional conformation of the AHAS-inhibitor complex predicts several amino acids in the proposed inhibitor binding pocket as sites where induced mutations would likely confer selective tolerance to imidazolinones (Ott et al., 1996, J. Mol. Biol. 263:359-368). Tobacco plants produced with some of these rationally designed mutations in the proposed binding sites of the AHAS enzyme have in fact exhibited specific tolerance to a single class of herbicides (Ott et al., 1996, J. Mol. Biol. 263:359-368).
Plant tolerance to imidazolinone herbicides has also been reported in a number of patents. U.S. Pat. Nos. 4,761,373, 5,331,107, 5,304,732, 6,211,438, 6,211,439, and 6,222,100 generally describe the use of an altered Als nucleic acid to elicit herbicide tolerance in plants, and specifically disclose certain imidazolinone tolerant corn lines. U.S. Pat. No. 5,013,659 discloses plants exhibiting herbicide tolerance possessing mutations in at least one amino acid in one or more conserved regions. The mutations described therein encode either cross-tolerance for imidazolinones and sulfonylureas or sulfonylurea-specific tolerance, but imidazolinone-specific tolerance is not described. Additionally, U.S. Pat. No. 5,731,180 and U.S. Pat. No. 5,767,361 discuss an isolated gene having a single amino acid substitution in a wild-type monocot AHAS amino acid sequence that results in imidazolinone-specific tolerance.
To date, the prior art has not described mutations in the Als1 gene that confer increased tolerance to an imidazolinone herbicide other than the mutation in the FS-4 imidazolinone tolerant line. Nor has the prior art described imidazolinone tolerant wheat or triticale plants comprising at least one altered Als nucleic acid from a Triticum aestivum Shiloh cultivar. Therefore, what is needed in the art is the identification of additional mutations that confer tolerance to imidazolinone herbicides. What are also needed in the art are wheat plants and triticale plants having increased tolerance to herbicides such as imidazolinone and containing at least one altered Als nucleic acid. Also needed are methods for controlling weed growth in the vicinity of such wheat plants and triticale plants. These compositions and methods would allow for the use of spray over techniques when applying herbicides to areas containing wheat plants and triticale plants.